Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
WDM systems use components referred to generically as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer divides an signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. A router works much the same way as a de-multiplexer. However a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port.
FIG. 1 depicts a typical optical interleaver 999 of the prior art as described in U.S. Pat. No. 5,694,233, issued to Wu et al. on Dec. 2, 1997, which is incorporated herein by reference for all purposes. A WDM signal 500 containing two different channels 501, 502 enters interleaver 999 at an input port 11. A first birefringent element 30 spatially separates WDM signal 500 into horizontal and vertically polarized components 101 and 102 by a horizontal walk-off. Component signals 101 and 102 both carry the full frequency spectrum of the WDM signal 500.
Components 101 and 102 are coupled to a polarization rotator 40. The rotator 40 selectively rotates the polarization state of either signal 101 or 102 by a predefined amount. By way of example, in FIG. 1 signal 102 is rotated by 90.degree. so that signals 103, 104 exiting rotator 40 are both horizontally polarized when they enter a wavelength filter 61.
Wavelength filter 61 selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals 105 and 106. For example wavelength filter 61 rotates wavelengths in the first channel 501 by 90.degree. but does not rotate wavelengths in the second channel 502 at all.
The filtered signals 105 and 106 enter a second birefringent element 50 that vertically walks off the first channel into beams 107, 108. The second channel forms beams 109, 110.
A second wavelength filter 62 then selectively rotates the polarizations of signals 107, 108 but not signals 109, 110 thereby producing signals 111, 112, 113, 114, having polarizations that are parallel each other. A second polarization rotator 41 then rotates the polarizations of signals 111 and 113, but not 112 and 114. The resulting signals 115, 116, 117, and 118 then enter a third birefringent element 70. Note that second wavelength filter 62 may alternatively be replaced by a polarization rotator 41 suitably configured to rotate the polarizations of signals 111, 113 but not 112, 114.
Third birefringent element 70 combines signals 115 and 116, into the first channel, which is coupled to output port 14. Birefringent element 70 also combines signals 117 and 118 into the second channel, which is coupled into output port 13.
As described above, interleaver 999 operates as a de-multiplexer. By operating interleaver 999 in reverse, i.e., starting with channels 501, 502 at ports 13 and 14 respectively, interleaver operates as a multiplexer. Furthermore, by suitably controlling the polarization rotation induced by rotators 40 and 41, interleaver 999 may be configured to operate as a router.
Interleaver 999 has certain drawbacks. First, each port requires its own collimator. Three collimators take up space and require a relatively large walk-off distance for the signals. Consequently, birefringent elements 30, 50 and 70 tend to be both long and wide. Second, the number of components, particularly birefringent elements, tends to make interleaver 999 bulky, expensive and more massive. Generally, the greater the mass of interleaver 999, the more unstable its operation. Third, the coupling distance, i.e., the distance between port 11 and ports 13, 14, tends to be long, which increases insertion losses in interleaver 999. Furthermore, each of the ports 11, 13 and 14 requires a separate collimator to couple the signals into and out of optical fibers. This adds the complexity and expense of interleaver 999.
There is a need, therefore, for an improved optical interleaver that overcomes the above difficulties.